1. Technical Field
The present invention relates to an electromechanical generator for converting mechanical vibrational energy into electrical energy and to a method of converting mechanical vibrational energy into electrical energy. In particular, the present invention relates to such a device which is a miniature generator capable of converting ambient vibration energy into electrical energy for use, for example, in powering intelligent sensor systems. Such a system can be used in many areas where there is an economical or operational advantage in the elimination of power cables or batteries.
2. Description of the Prior Art
There is currently an increasing level of research activity in the area of alternative power sources for micro electrical mechanical systems (MEMS) devices, such devices being described in the art as being used for ‘energy harvesting’ and as ‘parasitic power sources’. Such power sources are currently being investigated for powering wireless sensors.
It is known to use an electromechanical generator for harvesting useful electrical power from ambient vibrations. A typical magnet-coil generator consists of a spring-mass combination attached to a magnet or coil in such a manner that when the system vibrates, a coil cuts through the flux formed by a magnetic core. The mass which is moved when vibrated is mounted on a cantilever beam. The beam can either be connected to the magnetic core, with the coil fixed relative to an enclosure for the device, or vice versa.
In a paper entitled “Architecture for vibration-driven micropower generators”, by Mitcheson et al, published in the Journal of Micromechanical Systems, Vol. 13, No. 3, June 2004, pp. 335-342, various electromechanical generators are disclosed. In particular, a velocity-damped resonant generator (VDRG) is disclosed which consists of a damper for extracting energy from a mass-spring system. Such a damper may consist, for example, of a magnet-coil generator, such as the combination of two magnets mounted on a keeper to form a C-shaped core with a coil placed in the air-gap between the magnets at right angles to the direction of movement of the mass on a cantilever beam.
The authors identify a damping factor for determining the maximum power obtainable from the velocity-damped resonant generator. In particular, the authors provide a calculation for the optimal damping factor at which maximum power is obtained. The optimal damping factor is calculated using the resonant frequency of the velocity-damped resonant generator.
While this prior disclosure produced a useful mechanism for designing a theoretical electromechanical generator, when an electromechanical generator is used in a practical application, it is not possible accurately to predict the resonant frequency or the optimal damping factor. The electromechanical generator is designed and set up for what is believed to be the likely operating conditions. However, there is no guarantee that the practical operating conditions correspond to the theoretical ideal used to set up the electromechanical generator for the specific application. In practice, an electromechanical generator is set up to be operable across a narrow range of likely operating conditions, in particular with the damping factor being set up so that the power output is within a range encompassing the optimal power output. However, it is very unlikely that the actual power output is optimised for the specific application. Consequently, the electromechanical generator would not operate at maximum efficiency of the conversion of mechanical vibration energy into electrical energy, and thereby into useful electrical power.
Also, the frequency of ambient vibration may change during operation. The known electromechanical generator may not be able to operate at maximum efficiency as a result of such a change.
Yet further, the damper of the electromechanical generator incorporates a sprung mass that oscillates about a central position at a frequency intended to correspond to the resonant frequency to which the device is to be subjected in use. The amplitude of the resonant vibration depends upon a number of variables, in particular the frequency and magnitude of the driving vibration, the Q-factor of the resonator, the resonator mass and its resonant frequency.
These variables are not all predictable from the actual conditions encountered when the electromechanical generator is put into use in the field to harvest energy from a vibrating body. The amplitude of vibration of the sprung mass may vary with time, in an intermittent and unpredictable manner.
In particular, the electromechanical generator may, in use, be subjected to a vibration that causes the sprung mass to oscillate with excessively large amplitude (hereinafter referred to as an “unsafe” amplitude) with the result that the mass may physically impact upon the device's outer casing at the limits of its travel. Alternatively if there is no casing, then the mass may oscillate with an excessively large amplitude that causes permanent damage or degradation to the spring by exceeding the yield stress of the spring material. In either case, it may be expected that this impacting or yielding would reduce the operating lifetime of the electromechanical generator to an unacceptable degree.
In the case that the device has an outer casing that is being impacted, it may be possible to use a compliant material as a buffer between the surfaces that may mutually contact or impact. However, such a “mechanical stop” would be subject to wear and would not completely eliminate the extra stress to which the spring would be subjected on impact.
Resonant vibration energy harvesters are advantageously designed such that their Q-factor is as high as possible. This is because higher powers can be generated with higher-Q resonators. However if such a device is situated in an environment where the driving vibration becomes higher in magnitude than expected, then the amplitude of the resonator may become larger than the that designed or accommodated for. This amplitude may cause the resonating mass to impact on the device casing and potentially lead to permanent device damage after prolonged exposure.